The market for home networking is developing at a phenomenal rate. Service providers from cable television, telephony and digital subscriber line markets are vying to deliver bundled services such as basic telephone service, Internet access and entertainment directly to the consumer. Collectively these services require a high-bandwidth network that can deliver 30 Mbits/s or even higher rates. The Institute of Electrical and Electronic Engineers (IEEE) 802.11a standard describes a cost-effective, robust, high-performance local-area network (LAN) technology for distributing this multimedia information within the home. Networks that will operate in accordance with standard 802.11a will use the 5-GHz UNII (unlicensed National Information Infrastructure) band and may achieve data rates as high as 54 Mbits/s, a significant improvement over other standards-based wireless technology. The 802.11a standard has some unique and distinct advantages over other wireless standards in that it uses orthogonal frequency-division multiplexing (OFDM) as opposed to spread spectrum, and it operates in the clean band of frequencies at 5 GHz.
OFDM is a technology that resolves many of the problems associated with the indoor wireless environment. Indoor environments such as homes and offices are difficult because the radio system has to deal with a phenomenon called “multipath.” Multipath is the effect of multiple received radio signals coming from reflections off walls, ceilings, floors, furniture, people and other objects. In addition, the radio has to deal with another frequency phenomenon called “fading,” where blockage of the signal occurs due to objects or the position of a communications device (e.g., telephone, TV) relative to the transceiver that gives the device access to the cables or wires of the cable TV, telephone or internet provider.
OFDM has been designed to deal with these phenomena and at the same time utilize spectrum more efficiently than spread spectrum to significantly increase performance. Ratified in 1999, the IEEE 802.11a standard significantly increases the performance (54 Mbits/s vs. 11 Mbits/s) of indoor wireless networks.
The ability of OFDM to deal with multipath and fading is due to the nature of OFDM modulation. OFDM modulation is essentially the simultaneous transmission of a large number of narrow band carriers, sometimes called subcarriers, each modulated with a low data rate, but the sum total yielding a very high data rate. FIG. 1A illustrates the frequency spectrum of multiple modulated subcarriers in an OFDM system. To obtain high spectral efficiency the frequency response of the subcarriers are overlapping and orthogonal, hence the name OFDM. Each narrowband subcarrier can be modulated using various modulation formats such as binary phase shift keying (BPSK), quaternary phase shift keying (QPSK) and quadrature amplitude modulation (QAM) (or the differential equivalent).
Since the bandwidth rate on each subcarrier is low, each subcarrier experiences flat fading in multipath environment and is easy to equalize, where coherent modulation is used. The spectrums of the modulated subcarriers are not separated but overlap. The reason why the information transmitted over the carriers can still be separated is the so called orthogonality relation giving the method its name. The orthogonality relation of the subcarriers requires the subcarriers to be spaced in such a way that at the frequency where the received signal is evaluated all other signals are zero. In order for this orthogonality to be preserved it helps for the following to be true:                1. Synchronization of the receiver and transmitter. This means they should assume the same modulation frequency and the same time-base for transmission (which usually is not the case).        2. The analog components, part of transmitter and receiver, are of high quality.        3. The multipath channel needs to accounted for by placing guard intervals which do not carry information between data symbols. This means that some parts of the signal cannot be used to transmit information.        
If the receiver and transmitter are not synchronized in frequency the orthogonality of the subcarriers is compromised and data imposed on a subcarrier may be not be recovered accurately due to inter-carrier interference. FIG. 1B illustrates the effect of the lack of synchronization on the frequency spectrum of multiple subcarriers, The dashed lines show where the spectrum for the subcarrier should be, and the solid lines shows where the spectrum falls due to the lack of synchronization. Since the receiver and transmitter need to be synchronized for reliable OFDM communication to occur, but in fact in practice they are not, it is necessary to compensate for the frequency offset between the receiver and the transmitter. The offset can occur due to the inherent inaccuracy of the synthesizers and crystals in the transmitter and receiver and to drift due to temperature or other reasons. The offset can be compensated for at the receiver, but present methods only produce a coarse estimate of the actual offset. According to one method for compensating for the offset, the analog signal received by a receiver is divided into three sections: short timing symbol section, long timing symbol section and data symbol section. Some of the short timing symbols in the short symbol section are used for automatic gain control and for detecting symbol timing. Other short timing symbols are sampled and digitized and auto-correlated to produce a coarse estimate of the offset. The coarse estimate of the offset is then used to produce a digital periodic signal whose frequency is based on the coarse estimate of the offset. The digital periodic signal is multiplied with digital samples of the long symbols and the product is fast fourier transformed to produce a channel estimate. The digital carrier is also used to multiply digital samples of the data symbols (digital data samples) when they arrive, thereby correcting for the offset. The product of the digital carrier and the digital data samples can now be decoded.
Since the short symbols, from which the frequency offset was derived, are relatively short, the estimate of the offset may be off appreciably from the actual offset. Consequently, there will be a residual offset which may cause the spectrum of one subcarrier to overlap with the spectrum of another subcarrier. Due to the overlap, when the digital data samples are recovered the data for one subcarrier may include interference from an adjacent subcarrier, degrading the throughput of the communication system. Furthermore, since there is a residual offset, the channel estimate is not an accurate representation of the actual transfer function due to the channel.
As described above, existing solutions are not capable of providing a relatively good estimate of the frequency offset between a receiver and transmitter or channel estimate. Consequently, it is desirable to provide a solution that overcomes the shortcomings of existing solutions.